This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. In order to produce multi-pulse ESR sequences that will be used in our studies of molecular dynamics, we need to use high-power high-speed PIN diode switches for different microwave frequencies. Our prior work in this area serves as a starting point. PIN diode switches may be employed at the receiver to protect it from powerful RF pulses at high repetition rates and to ensure TWTA noise blanking during the periods of signal capture. Using an X-band waveguide switch, we have tested engineering samples of 250 V gallium arsenide and commercial 500 V silicon pin-diodes. These high-voltage diodes were installed in an X-band waveguide switch, based on a Varian VDX-1226 waveguide limiter switch, operating in a low-loss transmission configuration with an insertion loss of 0.5 dB and isolation of 78 dB. It features respective power levels of 200 W and 800 W for the two types of diodes. These power levels would be sufficient for the work with low-volume dielectric resonators designed for our experiments on molecular dynamics. A high-speed CMOS driver circuit, patented by C. Dunnam, was employed to bias the diodes. The pulse edge transition times were quite short for the gallium arsenide, where we produced pulses as narrow as 20 ns. We achieved 6-8 ns switching speed with these diodes and 10-20 ns with the silicon ones. The work on this switch will be continued with a goal to improve the data rate for silicon PIN diodes and to improve on power handling and switching noise with the gallium arsenide diodes. The new development in this area is directed towards providing multi-pulse capability at the level of several watts for 3D-imaging work. The timing system will generate CPMG with 100 ns inter-pulse spacing and switches will blank the receiver and cut transmitter noise entirely in less than 20 ns. Fast pin-diode switches with 80-100 dB of isolation were purchased.